(a) Except as provided in Sec. 725.361, no person other than the Secretary of Labor and authorized personnel of the Department of Labor shall participate at any stage in the adjudication of a claim for benefits under this part, unless such person is determined by the appropriate adjudication officer to qualify under the provisions of this section as a party to the claim. The following persons shall be parties:
(1) The claimant;
(2) A person other than a claimant, authorized to execute a claim on such claimant's behalf under Sec. 725.301;
(3) Any coal mine operator notified under Sec. 725.407 of its possible liability for the claim;
(4) Any insurance carrier of such operator; and
(5) The Director in all proceedings relating to a claim for benefits under this part.
(b) A widow, child, parent, brother, or sister, or the representative of a decedent's estate, who makes a showing in writing that his or her rights with respect to benefits may be prejudiced by a decision of an adjudication officer, may be made a party.
(c) Any coal mine operator or prior operator or insurance carrier which has not been notified under Sec. 725.407 and which makes a showing in writing that its rights may be prejudiced by a decision of an adjudication officer may be made a party.
(d) Any other individual may be made a party if that individual's rights with respect to benefits may be prejudiced by a decision to be made.